The present invention relates to automotive lubricants such as engine oils, gear oils and automatic transmission fluids.
Traditionally automotive lubricants have been based on conventional mineral oils. Whilst these have proved adequate in the past, mineral oil basestocks cannot always meet the increasing demands for superior lubricant properties, especially operational lifetime. These improved properties can be obtained to some extent by the use of additives, but research has also been conducted into modifying or changing the basestocks. In recent years lubricant manufacturers have produced automotive lubricants based on synthetic basestocks, for example polyalphaolefins and esters. Whilst these provide improved performance, they have the disadvantage that they are expensive.
There is therefore a need for an automotive lubricant with an alternative, less expensive basestock which provides improved properties.
UK Patent 737,392 discloses a lubricating oil containing an organo-tin compound as antioxidant. The basestock may be derived from petroleum distillates and residuals refined by conventional. means, hydrogenated mineral oils, white mineral oils, or polyether and polyester lubricants used alone or blended with mineral oil lubricants. The lubricant may be applied as a crank case oil, heating oil, hydraulic fluid, cutting oil, turbine oil or transformer oil.
U.S. Pat. No. 3,853,773 discloses an anti-gum and solvating lubricant for use with precision mechanical devices. It is based on a combination of a type-A transmission fluid (as defined in. the patent) with a highly refined white oil.
U.S. Pat. No. 4,652,385 discloses a lubricant containing a combination of a tri-substituted phosphite and sterically hindered phenolic stabilisers as antioxidant.. The basestock is a hydrotreated oil, polyalphaolefin oil or paraffinic white oil, or mixtures thereof. The lubricant is used in high temperature applications, for example as a compressor oil, heat transfer oil, hydraulic fluid or steam turbine oil.
The present invention provides an automotive lubricant comprising:
(a) a basestock of which at least 30 wt. % is a white oil; and
(b) an antioxidant selected from one or more of zinc dialkyldithiophosphate, zinc diaryldithiophosphate, zinc alkylaryldithiophosphate, alkylated diphenylamine, hindered phenol, phosphosulphurised alkylphenol, sulphurised phenol, dimercapto-dithiadiazole and copper-based antioxidant compounds;
with the proviso that the lubricant does not contain a tin-containing antioxidant or an organically substituted phosphite or diphosphite antioxidant.
The white oil based automotive lubricant according to the invention has the advantage that it possesses superior oxidation stability properties compared with automotive lubricants based on mineral oils, but has a lower production cost compared with lubricants based on synthetic oils. Thus the lubricant has the benefit of increased operation times, i.e. it can be used to lubricate a mechanical device, for example an engine or gear box for an extended period before it requires draining and replacing. In some applications the lubricant can be used as a fill-for-life lubricant, i.e. the operational life-time of the lubricant matches or exceeds that of the mechanical part it is lubricating. White oils are defined in the xe2x80x9cFood and Drug Administration Code of Federal Regulationxe2x80x9d, 1991. Either medical white oils according to specification FDA 21 CFR 178-3620 (a) or technical white oils according to specification FDA CFR 178-3620 (b) may be employed in the invention.
The white oil is a conventional white oil, obtained using conventional solvent extraction and hydrogenation to produce saturated hydrocarbons free from sulphur and nitrogen. It has been found that white oils with a relatively high naphthenic content exhibit improved properties compared with more paraffinic white oils. Preferably the white oil, used in the present invention naphthenic content of at least 25 wt. %, where xe2x80x98naphthenic contentxe2x80x99 is defined as the amount of naphthenic carbon as a percentage of the total carbon content of the white oil, according to standard test ASTM D 2140. More preferably the naphthenic content of the white oil is from 30 to 50 wt. %, most preferably 30 to 40 wt. %. A highly naphthenic white oil is obtained by using mild hydrogenation conditions, so that the cyclic molecules contained in the oil are not substantially broken. Typical mild hydrogenation conditions are a temperature of between 150 and 250xc2x0 C., and a pressure 1000 and 20,000 kPa.
The naphthenic composition of the highly naphthenic white oils advantageously used in the present invention is preferably as follows, the measurements being obtained using standard test method ASTM D 2786:
1 ring: 20-30 wt. %, preferably 24-32 wt. %
2 rings: 13-27 wt. %, preferably 17-23 wt. %
3 rings: 4-21 wt. %, preferably 8-17 wt. %
4 rings: 3-19 wt. %, preferably 7-15 wt. %
5 rings or more: 0-9 wt. %, preferably 2-5 wt. %
Examples of suitable FDA regulation food grade quality white oils that can be used in the present invention include MARCOL 52xe2x80x94naphthenic content 34%, MARCOL 82xe2x80x94naphthenic content 32%, MARCOL 172xe2x80x94naphthenic content 34%, PRIMOL 352 xe2x80x94naphthenic content 32%, and PLASTOL 352xe2x80x94naphthenic content 32%, all supplied by Exxon/Esso. Examples of suitable FDA regulation technical grade white oils that can be used in the present invention include BAYOL 52xe2x80x94naphthenic content 34% and PLASTOL 135xe2x80x94naphthenic content 36%, both supplied by Exxon/Esso. MARCOL, PRIMOL, PLASTOL and BAYOL are trade marks of Exxon Corporation. The naphthenic content is measured according to standard test method ASTM 2140.
The basestock may comprise 100% white oil, or may comprise a blend of white oil with one or more other types of oil, for example a mineral oil and/or a synthetic oil such as a polyalphaolefin or an ester such as a polyol ester or diester, and/or a hydrocracked-type basestock. If the basestock is a blend, the preferred proportion of white oil in the basestock is at least 30 wt %, more preferably between 30 and 60 wt %, most preferably between 30 and 40 wt. %. If the white oil is blended with a synthetic oil, the synthetic oil is preferably a polyalphaolefin, for example PAO 4 and/or PAO 6, where 4 and 6 are the respective viscosities of the PAOs in centistokes at 100xc2x0 C. Where the basestock is a blend of white oil, mineral oil and synthetic oil, the preferred proportions are 30-80 wt. % white oil, 10-70 wt. % mineral oil and 5-50 wt. % synthetic oil.
The automotive lubricant may also contain other additives such as those typically contained in an engine oil, gear oil or automotive transmission fluid as appropriate. These include detergents, for example alkaline earth metal sulphonates, calcium salycilates, alkaline earth metal sulphurised phenates; ashless dispersants, for example polyisobutenesuccinimide, anti-wear/extreme pressure agents, for example zinc dialkyl (or diaryl or arylalkyl) dithiophosphate, and phosphorus/sulphurous or borated compounds; anti-corrosion agents, for example barium alkylnaphthalene sulphonates and mercaptobenzotriazole; viscosity index improvers, for example olefin copolymers, polyalphaolefinst polymethacrylates and styrene butadiene; pour point depressants, for example polyesters; anti-foam agents, for example those based on silicon; and friction modifiers, for example molybdenum compounds, ashless compounds and anti-squawk agents. For each additive, the amount included in the automotive lubricant varies depending upon the type of additive and the intended application of the lubricant. Generally, however, each additive is added in an amount up to 6 wt % based on the total weight of the lubricant except for the viscosity index improver(s) which may be added in an amount up to about 10 wt % (active ingredient). Some or all of the additives may be incorporated into the automotive lubricant by means of an addpack.
In general terms, the automotive lubricant according to the invention has a viscosity of 4 to 50 mm2/s at 100 xc2x0 C., and a viscosity index of 80 to 200. More specifically, where the lubricant is an engine oil, it preferably has a viscosity of 4 to 35 mm2/s, more preferably 5 to 25 mm2/s, at 100xc2x0 C., and a viscosity index of 85 to 160, more preferably 95 to 150. Where the lubricant is a gear oil, it preferably has a viscosity of 5 to 50 mm2/s, more preferably 8 to 25 mm2/s, at 100xc2x0 C., and a viscosity index of 80 to 180, more preferably 95 to 160. Where the lubricant is an automatic transmission fluid, it preferably has a viscosity of 4 to 10 mm2/s, more preferably 5 to 8 mm2/s, at 100xc2x0 C., and a viscosity index of 100 to 200, more preferably 150 to 200.
It is important that the white oil contains an antioxidant additive. Surprisingly, it has been found that the white oil tested without the addition of an antioxidant is sensitive to oxidation and can have a lower performance than mineral oil. However, when an anti-oxidant is included in the white oil lubricant formulation the oxidation performance is superior to a comparable formulation based on mineral oil.
The antioxidant is selected from one or more of zinc dialkyl dithiophosphate, zinc diaryl dithiophosphate, zinc alkylaryl dithiophosphate, alkylated diphenylamine, hindered phenol, phosphosulphurised alkylphenol, sulphurised phenol, dimercapto dithiadiazole, and copper based antioxidants such as copper oleate and copper polyisobutylene succinic anhydride or a derivative thereof. The amount of antioxidant added to the lubricant is preferably from 0.05 to 3 wt %, more preferably from 0.1 to 2 wt %, based on the total weight of the lubricant, and most preferably from 0.2 to 1.0 wt %.
The white oil of the gear oil or automatic transmission fluid is preferably as described above, and may be blended with mineral oil or synthetic oil or both, to form a blended white oil basestock. If blended, the basestock preferably comprises at least 50 wt % white oil based on the weight of the basestock.